The present invention generally relates to communication control arrangements for spacecraft such as geosynchronous satellites, and more particularly, to an improved offset pointing arrangement for de-yawed phased array antennas.
Generally, satellites are positioned in geosynchronous orbit about the Earth to provide a relay for signal transmissions between different Earth locations. Angular offsets between the antenna of a space based communication system and the coordinate system of the satellite/host spacecraft will cause the antenna beam pattern to be improperly pointed.
In the past, various mechanical arrangements have been utilized to keep the satellite antenna coverage pattern aligned with a specific region of the Earth. Such systems typically provide physical biasing of the satellite attitude so as to compensate for any misalignment between antenna coverage and the desired satellite coordinate frame. Such physical biasing requires movement of the entire satellite.
However, movement of the entire satellite/spacecraft by any of a plurality of inertial techniques undesirably depletes the stored energy of the satellite. This in turn leads to an increase in the amount of fuel required to fire the appropriate attitude thrusters on the space craft. Thus, the use of physical movement control in order to provide antenna offset pointing decreases mission life and increases ground support logistics for on-orbit operation management.
Other arrangements have been proposed which employ a phased array antenna arrangement which can be electronically xe2x80x9csteeredxe2x80x9d to provide offset pointing compensation. While such an arrangement eliminates the need for physically biasing the spacecraft, the size and capabilities of on-board processing electronics are significantly increased and made more complex, thereby increasing the cost and payload of the spacecraft.
Electronic angular offset compensation using a phased array antenna is further complicated for yaw steered spacecraft which require concomitant de-yawing of the antenna beams to minimize movement of the spot beams relative to the desired coverage area. More specifically, one such antenna de-yawing control is implemented using on-board processing of uplinked control parameters calculated by ground control to provide electronic offset correction of a phased array antenna arrangement. The combined needs for processing de-yawing commands and offset pointing commands further complicates and enlarges the on-board processing electronics.
Therefore, a need exists for a satellite/spacecraft antenna offset pointing control arrangement which compensates for the spacecraft motion and/or orbital drift by maintaining the antenna pattern precisely pointed at a desired Earth coverage area which does not require physical biasing of the satellite, or significantly increase on-board processing requirements, particularly for de-yawed spacecraft.
It is therefore an object of the present invention to provide a method and system for control of offset pointing of a phased-array antenna which reduces on-board processing complexity and obviates the need for physical biasing of a spacecraft.
It is another object of the present invention to provide a method and system for control of offset pointing of a phased-array antenna mounted on a spacecraft which allows reduces the number of necessary on-board computations using a set of control parameters uplinked from a ground controller.
It is yet another object of the present invention to provide a method and system for control of offset pointing of a phased-array antenna which reduces on-board processing complexity for de-yawed spacecraft.
In accordance with these and other objects, the present invention provides a method and system for controlling offset pointing of a phased array antenna arrangement mounted on a spacecraft which controls electronic pointing compensate for any deviations in desired antenna coverage relative the Earth, thereby eliminating the need for physical movement of the spacecraft. The offset pointing control is accomplished via a set of steering control parameters which allow offset compensation even if the spacecraft is physically reoriented, i.e., de-yawed, with respect to the Earth. The control parameters are preferably uplinked for minimal final processing by an on-board processor and beam controller.
More specifically, a beam controller, which is responsive to set of control parameters transmitted by a remote ground command station, is controlled via an on-board processor to electronically steer the phased array antenna to correct for detected orbital drift. Such drift can be measured by use of spacecraft-based sensor arrangements and relayed to a ground control station. The uplinked control parameters reduces on-board processor complexity while reducing spacecraft fuel consumption typically expended in correcting for such drift.
Thus, in accordance with the present invention, a phased array antenna is used to reduce or eliminate pointing error by modifying the phased array steering commands in a way that compensates for the offsets. The present invention, moreover, does so even on yaw steered spacecraft whose antenna beams must be de-yawed to minimize the movement of spot beams pointed to the surface of the Earth. In particular, the present invention achieves electronic offset pointing on de-yawed phased array antennas with only minimal changes required to the on-board processing requirements when compared to a de-yawing control arrangement alone.
The present invention makes it possible to compensate for misalignments between a spacecraft and a communication payload antenna without physically biasing the spacecraft attitude. Since most space based communication systems employ Earth sensors for attitude measurement, and since the accuracy of Earth sensors degrades when they are operated off null, the present invention eliminates a significant source of pointing error. In addition, the present invention simplifies the on-board processing required to achieve electronic offset correction on de-yawed phased array antennas by implementing some of the required computations off-line on the ground. By application of the invention, only two additions per antenna beam per beam steering computational cycle are required for simultaneous electronic offset compensation and beam de-yawing when compared to de-yawing alone.